Final answer:
The suffix for "hardening" is -sclerosis, which is used in terms such as arteriosclerosis to denote the abnormal hardening of tissue.
Step-by-step explanation:
The suffix that denotes "hardening" is -sclerosis. In medical terminology, this suffix is used to indicate an abnormal hardening of tissue. For example, arteriosclerosis refers to the hardening of the arteries. The other options listed are different medical suffixes: -megaly indicates enlargement, -osis suggests a condition or disease, often one that involves an increase in cells, -ectasis means dilation or expansion, and -itis signifies inflammation.